The present invention relates generally to a communication system and method with transmittal signal encoding and particularly to an orthogonal communication method employing orthogonal encoding.
Deliberate bandwidth expansion, by redundant coding, is presently employed because of the advantage it confers on the parameters of performance. However, this advantage can be lost if the communication channel suffers from delayed echoes, time-dispersion or multipath effects.
Code Division Multiple Access, or CDMA, is a known technique often proposed to artificially widen transmission bandwidths. CDMA is an extension of well known redundant coding techniques such as the technique of repeat transmissions with majority vote at the receiver to combine signal repeats. In some applications of CDMA, also known as Direct Sequence Spread Spectrum, a mixture of simple repeats, or “dumb spreading,” and error correction coding, or “intelligent spreading,” is employed to achieve desired bandwidth widening ratio.
It is known in the prior art that it is advantageous to use less intelligent coding and to substitute an element of dumb spreading in such a way that different signals become orthogonal to one another and then do not interfere with each other. For example, if one signal after a suitable amount of intelligent error correction coding yields a coded bit stream a1,a2,a3,a4 . . . and a second signal yields a coded bit stream b1,b2,b3,b4.
Then the first signal is transmitted using additional four-times repeat coding as a1,a1,−a1,−a1,a2,a2,−a2,−a2,a3,a3,−a3,−a3,a4,a4,−a4,−a4 . . . while the second signal is transmitted with four times repeat coding as b1,−b1,−b1,b1,b2,−b2,−b2,b2,b3,−b3,−b3,b3,b4,−b4,−b4,b4 . . . , then a comparison of the sign pattern of the repeat coding ++−−++−−++−−++−− . . . for the first signal and the sign pattern of the repeat coding +−−++−−++−−++−−+ . . . for the second signal, shows that these differ in sign in exactly half the positions while agreeing in the other half. Thus, upon combining the repeats with the proper signs for enhancing one signal, the contribution from the interfering signal completely cancels, and vice versa. These signals are known as “mutually orthogonal.”
The U.S. digital cellular IS95 system specifies mutual orthogonality for transmissions from cellular base stations to mobile phones, using 64-fold repeat coding with one of 64 sign patterns selected from a set of 64 mutually orthogonal Walsh-Hadamard codes. The IS95 system uses non-orthogonal transmission in the direction from mobile phone to cellular base stations, using instead intelligent error correction coding comprising convolutional encoding concatenated with orthogonal Walsh-Hadamard block coding. In the mobile-to-base direction, the orthogonality between different Walsh-Hadamard codes is used to discriminate between different 6-bit symbols transmitted from the same mobile phone, while in the base-to-mobile direction, the Walsh-Hadamard codes are used to discriminate between symbols transmitted to different mobile phones.
A disadvantage of the IS95 system of non-orthogonal transmissions in the mobile-to-base direction is that these signals interfere with one another if the power of the mobile transmitter is not strictly controlled as a function of distance from the base station such that signals from different mobile phones are received at more or less the same power level. However, the need for strict power control is alleviated when practicing the invention disclosed in U.S. Pat. No. 5,151,919 issued to Dent on Sep. 29, 1992, entitled CDMA Subtractive Demodulation. In U.S. Pat. No. 5,218,619 issued to Dent on Jun. 8, 1993, entitled CDMA Subtractive Demodulation, already decoded signals are subtracted more than once to improve interference subtraction. U.S. Pat. No. 5,353,352 issued to Dent and Bottomley on Oct. 4, 1994, entitled Multiple Coding for Radio Communications, describes optimum spread spectrum access codes, equivalent to the sign patterns discussed above, when orthogonal signaling is employed within one transmission with non-orthogonality between different transmissions, such as used in an IS95 uplink in the mobile-to-base direction. The disclosures of the above referenced patents are hereby incorporated by reference here in their entirety.
The reason for the difference between IS95 uplink, (mobile-to-base), and IS95 downlink, (base-to-mobile), transmission schemes is that maintaining orthogonality between different transmissions requires that they be accurately aligned in time, when the prior art communication schemes are used. If, in the above example, the first and second signals are aligned with one another with a one place shift they are shown as follows:                ++−−++−−++−−++−−                    +−−++−−++−−++−−+The two exemplary sign patterns given above are seen now to differ only at the beginning and the end of symbol blocks, thus severely compromising orthogonality.                        
In the downlink, or base-to-mobile, direction all signals originate at the same base station and thus time alignment can be assured. When signals in the uplink or mobile-to-base direction originate at different mobile phones that lie at different distances from the base station, it is much more difficult to achieve time alignment of the signals received at the base station.
The European cellular system known as GSM employs dynamic time alignment of mobile transmissions, wherein individual mobile phones are commanded by a base station to advance or retard their timing to bring the signals received into a desired time relationship with one another. However, the ability to achieve such synchronization to a high accuracy, for example within fractions of a microsecond, is limited by multipath signal propagation phenomenon which is a characteristic of the land-based mobile radio environment.
The multipath signal propagation phenomenon is caused by reflections of transmitted signals from large objects such as hillsides and tall buildings, giving rise to delayed echoes. While it may be possible to synchronize signals transmitted from a mobile transmitter such that a selected signal ray or echo is time aligned and thus orthogonal to a ray from another mobile transmitter, multipath propagation, reflected rays, or echoes, with path delays different from those of the selected signal rays will not be time aligned.
The GSM system uses Time Division Multiple Access (TDMA) in which each mobile signal is allocated a timeslot that does not overlap with transmissions from other mobiles on the same frequency. A guard time between slots equal to the longest normally expected echo delays, plus the use of commanded time advance/retard, reduces interference between different transmissions caused by multipath propagation. The interference of an echo with its original signal has been reduced by using an equalizer that beneficially adds together energy in different echoes of the same signal. One such equalizer is described, for example, in U.S. Pat. No. 5,331,666 issued to Dent on Jul. 19, 1994, entitled Adaptive Maximum Likelihood Demodulator and U.S. Pat. No. 5,335,250 issued to Dent, et al. on Aug. 2, 1994, entitled Method and Apparatus for Bidirectional Demodulation of Digitally Modulated Signals, the disclosures of which are hereby incorporated by reference herein. The need for a guard time between time slots reduces the bandwidth capability of the systems while use of an equalizer does not eliminate all potential multipath propagation problems.
A need therefore still exists for a system and method that constructs and communicates signals that remain largely orthogonal to each other even when delayed by different amounts of time due, for example, to multipath propagation phenomenon.